The terahertz (THz) electromagnetic spectrum has oftentimes been recognized as an important region for scientific research. However, due to the lack of devices, circuits and systems for effective THz signal generation, detection, and modulation, this region remains at least one of the least explored and developed of the electromagnetic spectrum. The past decade witnessed an increase in THz research activities. Electrically tunable THz modulation is one of the actively pursued subjects due to its importance in applications such as communications, imaging, and spectroscopy.
In one example a THz modulator operates at room temperature by employing a semiconductor two-dimensional electron-gas (2DEG) structure. More particularly, in the example THz modulator, the modulation is achieved by controlling the electron density in a gated two-dimensional electron gas structure. For instance, reducing the electron density which leads to an increase in the transmitted intensity of an incident beam of THz radiation. By depleting an electron gas of density 1012 cm−2, the example THz modulator achieved a maximum modulation depth of approximately 3% of the THz electric field, or equivalently 6% modulation depth of the THz intensity, for a pulse of terahertz radiation covering the range of frequencies from 0.1 to 2 THz. Though room-temperature operation is an advance compared to previous devices that required operation at cryogenic temperatures, the intensity modulation depth (MD) of approximately 6% is oftentimes insufficient for practical use.
Consequently, one example of room-temperature THz modulator research has trended towards metamaterial approaches. For example, U.S. Pat. No. 7,826,504 describes a metamaterial structure for the modulation of terahertz frequency signals. In the described device, each element within an array of metamaterial elements comprises multiple loops and at least one gap. The metamaterial elements include resonators with conductive loops and insulated gaps, or the inverse in which insulated loops are present with conductive gaps; each providing useful transmissive control properties. The metamaterial elements are fabricated on a semiconducting substrate configured with a means of enhancing or depleting electrons from near the gaps of the metamaterial elements. A described on to off transmissivity ratio of about 0.5 is purportedly achieved with the example device described. It will be appreciated, however, that these metamaterial-based devices have several comparative disadvantages. For example, known metamaterial-based devices are intrinsically narrowband and usually have a polarization-dependent response.
Accordingly, while the previous devices generally work for their intended purposes, there is an identifiable need for an improved terahertz wave amplitude modulator as presently disclosed.